1. Field of Invention
The present invention relates to a transistor. More particularly, the present invention relates to a thin film transistor with an etching stop structure.
2. Description of Related Art
Currently, a top gate structure (device) of a conventional thin film transistor has been widely applied in a flat display apparatus. Herein, the top gate is directed to a gate disposed above a semiconductor stack layer which includes a polycrystalline silicon layer and an ohmic contact layer, i.e., a polycrystalline silicon layer is first formed and then an ohmic contact layer is formed above the polycrystalline silicon layer, and the ohmic contact layer has an N+ region or a P+ region therein. However, in a general process of manufacturing a general top gate transistor device, when the ohmic contact layer is etched, i.e., when the N+ region and P+ region are etched, the polycrystalline silicon layer right below and around the gate is liable to be etched, and thus a front path of the polycrystalline silicon layer (also referred to as an active layer) is damaged, that is to say, the polycrystalline silicon layer below the gate is etched to be thinner than the originally-designed thickness. When the polycrystalline silicon layer (also referred to as the active layer) of the transistor is thinner, it is more difficult to control the thickness uniformity of the polycrystalline silicon layer (also referred to as the active layer) in the etching process, thus causing the electrical property to be not uniform. Moreover, the damage of the front path will result in the decrease of the electron mobility, the increase of the current leakage and the deterioration of the subcritical effect, so that the reliability of the transistor device is not good and the top gate structure (device) of the thin film transistor is limited in its applications.
Therefore, the conventional structure of the top gate transistor device needs improvements.